Flavor asymmetry in the light-quark sea of the nucleon

Parton distributions with an excess of down quarks over up quarks in the sea can reproduce data on the structure functions $F_2^{\mu p}$, $F_2^{\mu n}$, and $x{\mathcal{F}}_3^{\nu N}$. A model calculation in chiral field theory shows how an up-down asymmetry can arise from the dissociation of a quark into a quark plus a pion within the nucleon. This effect is large enough to account for the Gottfried-sum-rule defect reported by the New Muon Collaboration. Similar calculations may advance the understanding of other quasistatic properties of hadrons.